Did some extensive googling and burrowing through various forums and found out CPID validity problems are a side effect of missing key-pair information. If you transfer your wallet.dat file to a new computer with a fresh install of the Gridcoin wallet, make sure you take the .conf file with it from the %appdata% subfolder. This will contain the public and private key associated with your CPID and will satisfy the wallet software for the beacon you already sent from the previous install. Apparently you can have a beacon removed and send another (presumably this generates new keys) but so far it looks like my invalid CPID issue is resolved.